Mars Express radar on hold

An experiment designed to look for water deep under the martian surface is in limbo after an antenna on the orbiting Mars Express probe unfolded wrongly.

MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) is the first experiment designed to find water several kilometres below the martian surface. But its deployment has been on hold since April 2004, about four months after Mars Express arrived at the red planet, because mission engineers feared that its radio antennae might hit other instruments on the craft when they swung into position.

After careful computer simulations of the manoeuvre, the first of the radio booms was finally unfolded from the European Space Agency craft on 4 May. But three days later, flight engineers at the European Space Operations Centre in Darmstadt, Germany, realized that one of the 13 segments had locked into position at an angle, putting a kink in the antenna.

ESA has delayed unfurling a second boom until they work out exactly what has gone wrong. Until at least two of MARSIS's three booms are correctly in place, the experiment is useless.

"It's frustrating," says Martin Siegert, of the University of Bristol, UK, who is one of the many scientists eagerly awaiting results from MARSIS. Siegert researches subsurface water in Antarctica, which might be an environment similar to underground reservoirs on Mars. "If there's water down there, it may be a habitat for life," he says.

Radio signals

Scientists hope that MARSIS will eventually use two 20-metre-long antennae to beam radio waves at Mars. Most will bounce off the surface, but some should penetrate several kilometres deeper, and reflect off any underground layers of liquid water or ice. Although the two booms alone can detect these reflected waves, a third, 7-metre-long boom is intended to pinpoint exactly where the waves are reflected from.

"We had lots of evidence of ancient water on Mars, and good evidence of ice, but no ability to detect buried liquid water before now," says Jeffrey Plaut, a geologist at NASA's Jet Propulsion Laboratory in Pasadena, California.

The booms have been packed, concertina fashion, since the probe left Earth in June 2003. It's too early to say whether keeping the boom folded for a year longer than expected has caused this problem, "but it is the only variable we cannot simulate here on Earth", admits Fred Jansen, mission manager for Mars Express. "We have run a number of simulations but this particular failure case has never come up," he told news@nature.com.

They could open the second boom now, says Jansen. But if that too locks into position at an angle, it could fatally destabilize the whole craft. At the moment, the risk seems too great to try that, he adds. So if ESA engineers cannot find a way to unlock the jammed hinge, the experiment may have to be abandoned.

UPDATED - 11 May 2005

The first MARSIS radar boom has now successfully locked into place, ESA says. Mission engineers guessed that the problem might have been caused by prolonged storage in the deep cold of space, which may have badly affected materials in the folded antenna. So they turned the Mars Express spacecraft around on 10 May, allowing the Sun to heat up the cold side of the boom for an hour. This appears to have expanded the joint enough to let the antenna pop into position. The deployment of the other two booms should continue in a few weeks, ESA says.